Fluoroelastomers are well known in the art. Many are copolymers of vinylidene fluoride (VF2) with at least one other fluorinated comonomer such as hexafluoropropylene (HFP), tetrafluoroethylene (TFE), and a perfluoro(alkyl vinyl ether). Other fluoroelastomers include copolymers of tetrafluoroethylene with a hydrocarbon olefin such as ethylene or propylene and copolymers of tetrafluoroethylene with a perfluoro(alkyl vinyl ether).
It is sometimes desirable to modify the properties of fluoroelastomers by grafting molecules onto the main polymer chains or onto side chains. The grafted molecules have pendant groups that can be employed for further modification, crosslinking, or for changing the physical properties (e.g. Tg) of the fluoroelastomer.
A typical graft involves the reaction of carbon-carbon double bonds on the fluoroelastomer with a grafting agent that has a single nucleophilic group (e.g. a “monophenol”) for reacting with the double bond. An agent having more than one nucleophilic group could result in crosslinking of fluoroelastomer chains and is not desirable. Grafting agents typically have one or more pendant groups such as a vinyl, allyl, acrylate, alkoxysilane, amido, sulfonic acid salt, pyridine, carboxylic ester and carboxylic salt group for modifying the fluoroelastomer properties. The carbon-carbon double bonds on the elastomer may be present prior to when the elastomer is mixed with the grafting compound, or the double bonds may be generated by a dehydrofluorination catalyst (i.e. an accelerator) in the presence of the grafting compound.
Unfortunately, the grafting process often results in an undesirable pre-vulcanization (i.e. increase in Mooney viscosity) of the fluoroelastomer composition that adversely affects subsequent processing, causing the composition to be difficult to mix, extrude or mold. The latter is sometimes referred to as “scorch”. This scorch phenomenon is a particular problem when the grafting reaction takes place in the bulk with a substantially dry fluoroelastomer gum.
Caporiccio et al. (U.S. Pat. No. 5,367,023) performed grafting in a dispersion or solution, rather than in the bulk with a substantially dry fluoroelastomer gum. However, the resulting grafted fluoroelastomer composition may still exhibit undesirable pre-vulcanization during further processing such as mill mixing, extruding or molding.
Halladay et al. (U.S. Pat. No. 6,838,407 B2) disclose a room temperature curable fluoroelastomer composition. The fluoroelastomer is grafted in the bulk with an agent having an active hydrogen-bearing group for subsequent crosslinking with an isocyanate. Grafting is preferably through an amino group or through a mercapto group. Such grafted fluoroelastomer compositions may still exhibit undesirable pre-vulcanization during further processing such as mill mixing, extruding or molding. Furthermore, amino and mercapto group grafts are undesirable in some end use applications because of the relatively poor hydrolytic stability of imine bonds resulting from amino group grafting and the limited thermal stability of sulfide bonds resulting from mercapto group grafting.
An object of the present invention is to make a monophenol grafted fluoroelastomer in the bulk, substantially dry state, wherein both the graftable composition and the resulting grafted fluoroelastomer have improved resistance to thermally induced pre-vulcanization.